Conventionally, a spark plug has been used for an internal combustion engine. As the spark plug, for example, a spark plug that includes a center electrode, an insulator, a metal shell, and a packing has been used. The center electrode extends in an axial line direction. The insulator has an axial hole extending in the axial line direction. The center electrode is arranged at a front end side of the axial hole. The metal shell is arranged at an outer periphery of the insulator. The packing is arranged between the insulator and the metal shell. As the insulator, for example, an insulator that includes a step part and an insulator nose portion has been used. The step part has an outer diameter reduced to the front end side. The insulator nose portion extends to the front end at the front end side of the step part. The packing is sandwiched between the step part of the insulator and the metal shell. Here, there has been proposed the following technique. To reduce breakage of the insulator, a curved surface portion is disposed between the step part of the insulator and the insulator nose portion. In addition to the step part of the insulator, the packing is also brought into contact with a site at the front end side with respect to an intermediate portion of the curved surface portion.